When K-1 was in its heyday the winner got $400,000 or $500,000 plus paid for every fight of the tournament. When Masato was king, they built the whole K-1 Max promotion around him and i'm sure he was getting well over a million a fight the draw he was. But top boxers make way more then any other fighters, at least for the time being.
As for MMA I only know payouts from the U.S. When Japan was booming they never released to the public the payouts, but fighters always said they payed well.
Canelo is a top guy, of course he is getting that kind of money. But your average boxer don't make nothing special.
Of course a young phenom in Tenshin could beat a 41 year old well past his prime Mayweather, all odds are overwhelmingly with Mayweather, but a fight is a fight, and anything can happen, that's why we watch.
K1 is financially crippled and has largely been replaced by Glory, Kunlun and to a lesser extent Bellator Kickboxing.
And no, Tenshin could not beat Floyd Mayweather.
Hell, Tenshin could probably bank 10 million if he fought Amir Khan, who is several orders of magnitude worse than any version of Floyd. And if they fought, Khan would box fucking circles around Tenshin, even with his glass jaw.
"The absolute top guys in a kickboxing promotion that is essentially bankrupt used to make comparable money to basic cable B side boxers"
"I'm sure undefined fighters working for undefined Japanese promotions 20 years ago when MMA was significantly less popular got paid bank"
Your "average" boxer may not make boatloads of cash. But an elite boxer makes more than any other athlete in the world. Conor McGregor is a household name in MMA but he makes about the same per fight as Dillian fucking Whyte, and ain't nobody who isn't a hardcore sports fan knows who Dilian Whyte is.
A main event boxer makes more than a main event MMA or kickboxer.
A mid level boxer makes way more than a midlevel MMA or kickboxer.
A journeyman boxer makes way more than a midlevel MMA or kickboxer.
It really doesn't get any more simple than this:
If Tenshin could beat a great boxer, he'd do it. He'd do it because a single fight against even a semi-famous boxer will make him more money than his entire kickboxing and MMA career combined so far.